A thin film transistor (TFT) formed over a flat plate such as a glass substrate, which is typically used in a liquid crystal display device, is generally formed using a semiconductor material such as amorphous silicon or polycrystalline silicon. Although a TFT which is formed using amorphous silicon has low field-effect mobility, such a TFT can be formed over a larger glass substrate. On the other hand, a TFT formed using polycrystalline silicon has high field-effect mobility, but such a TFT needs a crystallization step such as laser annealing and is not always suitable for being formed over a larger glass substrate.
In view of the foregoing, a TFT formed using an oxide semiconductor as a semiconductor material has attracted attention. For example, Patent Documents 1 and 2 each disclose a technique in which a TFT is formed using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as a semiconductor material and used for a switching element in an image display device.
A TFT in which a channel formation region is formed in an oxide semiconductor can have higher electric-field mobility than a TFT formed using amorphous silicon. Further, an oxide semiconductor film can be formed at a temperature of 300° C. or lower by a sputtering method or the like, and a manufacturing process of the TFT formed using an oxide semiconductor is simpler than that of the TFT formed using polycrystalline silicon.
TFTs which are formed using such an oxide semiconductor are expected to be applied to switching elements included in a pixel portion and a driver circuit of a display device such as a liquid crystal display, an electroluminescent display, and electronic paper. For example, Non-Patent Document 1 discloses a technique by which a pixel portion and a driver circuit of a display device include TFTs formed using the above oxide semiconductor.
Note that the TFTs which are formed using an oxide semiconductor are all n-channel transistors. Therefore, in the case where a driver circuit is formed using a TFT formed using an oxide semiconductor, the driver circuit includes only n-channel TFTs (hereinafter also referred to as unipolar TFTs).    [Patent Document 1] Japanese Published Patent Application No. 2007-123861    [Patent Document 2] Japanese Published Patent Application No. 2007-096055    [Non-Patent Document 1] T. Osada, et al., SID 09 DIGEST, pp. 184-187 (2009)